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Could the North Atlantic Ocean Currents Be Slowing Down? -
October 30, 2016

Ocean currents, such as the Gulf Stream, are able to transport mass amounts of water around the globe. But did you know that these currents may be slowing down? The Atlantic Meridional Overturning Circulation (AMOC) is the main current that runs through the Atlantic Ocean. The AMOC is responsible for regulating many aspects of the ocean including: sea level, establishing the Gulf Stream which warms much of northern Europe, and transporting nutrients from the deep sea and supplying them to surface waters where they become more readily available.

The AMOC is operated by the water’s density and controlled by the salinity and temperature of the water, making the AMOC a thermohaline current. Cold, salty water is denser than warm, fresher water. As warm salt water flows northward, it loses heat to the atmosphere and becomes denser. This dense, cool water sinks to the ocean bottom. Then surface water from the tropics is pulled in to replace it. This pulling system is what drives the deep ocean currents (AMOC).

The AMOC has been in operation for a long time; however, there are indications that this process is slowing down. Models of future climate show that changes in water densities through warming and freshening of North Atlantic surface waters will likely lead to a reduction of the circulation. However, the extent of the weakening of this pattern is not known, but is the subject of intense research efforts.

The acidity of the ocean is also on an upswing. The ocean is responsible for absorbing a quarter of Earth’s carbon dioxide (CO2). This carbon dioxide mixes with the ocean’s surface creating carbonic acid (H2CO3). This addition of carbonic acid is what creates an increase of acidity within the oceans. As the ocean currents slow, global temperatures continue to rise and the ability to absorb carbon dioxide decreases. One may believe that this is a positive attribute to the slowing of ocean currents, but it actually contributes to climate change.

As global temperature continues to rise, so does the freshness of water, meaning the ocean is losing its “saltiness”. The two main contributors to this phenomenon are the melting of the Greenland ice sheets and the increase of rain in North America. While the ice sheets are melting, increased amounts of freshwater are being released into the ocean. This phenomenon is expected to happen; however, the melting seems to be occurring at an increased rate and the ocean is having difficulty regulating itself. With the addition of freshwater through rain and glacial meltwater, the ocean is acquiring more and more freshwater.

The increase of freshwater and climate change is causing ocean currents, including the AMOC, to slow down. The pulling system (mentioned above) is beginning to slow. The outcome of this is that the “dense” water is becoming less salty because it is being diluted by the freshwater. A decrease in water density leads to a smaller pull of the current and less water that is transported around the globe. As a result of this, Europe and North America could become colder because the circulation has become too slow where not enough warm water from the tropics will reach them. This warm water instead will begin moving in a different direction, which could possibly end up increasing the numbers of hurricanes, Nor’easters, and storms along the AMOC pathway.

The slowing of the AMOC not only affects climate change and the saltiness of the ocean but it also has an effect on global food security. Without the AMOC working to full effect, deep sea nutrients are being withheld from surface oceanic systems that support important fisheries that thrive in the shallow Atlantic Ocean region. Without the proper nourishment, species will be negatively affected and population numbers will begin to decline. Another serious side effect of slowing currents is additional sea level rise. Water on the East Coast side of the AMOC is cooler than the water on the side facing Europe. As the AMCO slows down, this temperature difference will lessen. As the cool water off the coast warms up, it will also expand, causing an increase in sea level rise. In 2009 and 2010, the East Coast experienced a 4-inch rise in sea level attributed to the slowing of the AMOC.

As the large ice sheets, such as the Greenland and the Antarctic ice sheets continue to melt, the side effects of slowing ocean currents will certainly become more noticeable. Although sea level changes, along with changes in carbon dioxide levels, have occurred numerous times in our earth’s history, the speed of these recent changes is something the earth has not seen. Understanding the basis of climate change is the first step in finding the solution and the path to resilience.

LeCompte is an AmeriCorps member with the Maryland Coastal Bays Program.